A Zeolitic Imidazolate Framework (ZIF-8) Film for H2/CO2 Separation

A Zeolitic Imidazolate Framework-8 (ZIF-8) Film for H₂/CO₂ separation

Eunhee Jang¹, Jungkyu Choi^1*

¹ Department of Chemical & Biological Engineering, Korea University, Seoul 02841, Republic of Korea

ehphoenix@korea.ac.kr, jungkyu_choi@korea.ac.kr

Zeolitic imidazolate framework-8 (ZIF-8) membranes are highly suitable as H₂ selective walls in membranes reactors used for water gas shift reactions because of their efficient molecular sieving properties that favor H₂ transport, coupled with their high thermal stability and processability. In this study, we adopted an in-situ counter diffusion method (CD method) to fabricate a ZIF-8 membrane; Zn sources, already placed inside a porous support, were allowed to diffuse out and react with the 2-methylimidazole (mim) ligands in the bulk phase. Because the reaction rates between the Zn source and the mim ligand were very high, their diffusion rates played a key role in determining the final properties of the membranes. To exploit the effect of diffusion rate on preparation of ZIF-8 membrane via the CD method, a hierarchically structured support, i.e., a gamma alumina layer-coated alpha alumina disc (gamma/alpha alumina disc), was used in addition to an intact alpha alumina disc. Existence of ZIF-8 membrane was demonstrated with SEM, EDX of Zn atom, XRD, and contact angle. The position of ZIF-8 grains in the supports was estimated based on the direct measurement of N₂ physisorption. ZIF-8 membranes in the alpha alumina disc (membrane ZIF-8_A) were primarily formed on top similar to a conventional supported-membrane, whereas those in the gamma/alpha alumina disc (membrane ZIF-8_GA) were embedded inside the modified support. From the N₂ physisorption analysis, it was found that the ZIF-8 grains in membrane ZIF-8_GA was pre-dominantly formed inside the alpha alumina disc, while filling ~70% of the mesopores in the gamma alumina layer.  As desired, the embedded membrane ZIF-8_GA showed marked H₂ separation performance with a maximum H₂/CO₂ separation factor (SF) of ~9.9 plus-minus 1.2 at 250 deg C (vs. a maximum H₂/CO₂ SF of ~7.5 plus-minus 0.2 for membrane ZIF-8_A). Although both type membranes persisted at 200 and 250 deg C for up to 72 h, at a higher temperature of 300 deg C, the membrane performance started deteriorating after ~2 h and ~10 h for membranes ZIF-8_A and ZIF-8_GA, respectively. This indicates that the gamma alumina layer served as a protective layer for preserving the performance of the embedded ZIF-8 membrane. Their performance was completely degraded due to the eventual conversion of ZIF-8 into ZnO phases.

Figure. 1. H₂/CO₂ separation performance test of ZIF-8-A and ZIF-8-GA at 300 ¡ÆC for 72 h. Here, performance test result obtained for 48 h was expressed. Schematic images for ZIF-8-A and ZIF-8-GA was depicted at the bottom of the performance test.